Quadrature modulation is used in digital modulation techniques such as quadrature phase shift keying (QPSK), quadrature amplitude modulation (QAM), differential quadrature phase shift keying (DQPSK), etc. In systems where quadrature modulation is employed to transmit digitized voice or data, each data carrier, i.e., each I/Q modulated carrier, is created with its own set of radio frequency (RF) infrastructure including an I/Q modulator, a frequency synthesizer, a frequency up converter, and an RF power amplifier.
In many communication contexts, such as mobile radio communications, additional channel capacity is obtained by making more efficient use of the existing frequency spectrum allocated per communications channel. For example, in mobile radio communications, each communications channel is traditionally allocated 25 KHz of bandwidth. However, in newer digital mobile radio systems, bandwidth per channel is decreased so that two channels can be accommodated within the same 25 KHz, i.e., each channel occupies 12.5 KHz. With this more efficient use of bandwidth, two data carriers are transmitted where there once was only a single data carrier.
One approach to generate these two data carriers is for each carrier to be processed using their own dedicated RF infrastructure as shown in the transmitter for example in FIG. 1. Thus, a first set of data signals (I1, Q1) would be modulated in I/Q data modulator 12 using a first local oscillator frequency F.sub.101, typically an intermediate frequency (IF), generated by a first frequency synthesizer 14. The output of the I/Q modulator 12 is upconverted in mixer 16 using RF frequency F.sub.rf1 also generated by frequency synthesizer 14 before being amplified in RF power amplifier 18. The second set of data signals (I2, Q2) would be modulated in a second I/Q data modulator 20 using local oscillator frequency F.sub.102 generated by a second frequency synthesizer 22. The output signal from second I/Q modulator 20 would then be upconverted in a second mixer 24 using another RF frequency F.sub.rf2, the output of the mixer 24 being suitably amplified in a second RF power amplifier 26. The two amplified signals would then be transmitted over a single antenna using combiner 28.
Thus, in order for the transmitter in FIG. 1 to generate the two data carriers containing data signals (I1, Q1) and (I2, Q2), two sets of RF transmitter components such as two I/Q modulators, two frequency synthesizers, two RF power amplifiers, etc., are required. These RF components are expensive and/or bulky and consume considerable power. Thus, upgrading existing radios, such as base stations, to adapt to narrower channel spacing is a significant undertaking in terms of cost and hardware reconfiguration.
What is desired is a way in which two (or more) quadrature data carriers may be simply and efficiently combined before being modulated and transmitted so that there would be no need to duplicate expensive, bulky, and power consuming hardware components. It is therefore an object of the present invention to provide a transmitter which effectively combines two (or more) quadrature data signals before modulation in a single I/Q modulator.
For example, the present invention includes a combiner for combining plural data signals into a combined data signal. An I/Q modulator modulates the combined data signal which is then amplified in a single power amplifier and transmitted over an antenna. The combiner combines various components of the first and second data signals. The I/Q modulator may be a conventional I/Q modulator that quadrature modulates signals onto a single frequency carrier.
In one embodiment, the combiner in accordance with the present invention determines the sum and difference of various I/Q components of the two data signals. Summed outputs are modulated and phased shifted by 90.degree. using a low frequency sinusoidal signal and a mixer. Differenced outputs are mixed with the low frequency in-phase sinusoid, i.e., zero phase shift. The summed and differenced components respectively are summed to generate a single set of data signals (I', Q'). The I' signal is mixed in the I/Q modulator with a local oscillator intermediate frequency (IF), and the Q' component is mixed using the same local oscillator IF frequency shifted by 90.degree.. The IF in-phase and quadrature components are combined in an RF combiner and output to the single RF mixer and power amplifier components. Alternatively, a high frequency I/Q modulator could be used thereby eliminating the need for the RF mixer. The single RF output, therefore, contains two independent quadrature carriers.
The present invention also includes a method comprising the stops of converting two data signals each having first and second components into one data signal having a first and second component, and modulating the first and second components in an I/Q modulator. A single output of the I/Q modulator includes two independent quadrature modulated signals separated in frequency. The converting stop may further include the stops of summing certain ones of the first and second components; differencing certain ones of the first and second components, mixing summed differenced components with a low frequency signal; and combining the mixed signals to produce the one complex signal. As a result, the two independent quadrature modulated signals are separated by twice the frequency of the low frequency signal.
Thus, the present invention is advantageous in that a radio transmitter may be constructed to transmit two independent quadrature signals using only one modulator, one frequency synthesizer, and one power amplifier. These and other features and objects of this invention will become more apparent by reference to the following detailed description in conjunction with the accompanying drawings.